In a variable-frequency drive system, a DC bus voltage will increase if inverters are controlled in a way that energy flows from motors to DC links. Excessive DC bus voltage may be harmful to rectifier and inverter components, especially to capacitors within the circuits. As a result, the DC bus voltage may need to be limited below a threshold so as to avoid failures of various components.
One way to limit the DC bus voltage is to lead a portion of energy to a resistor associated with a braking chopper. The resistor is used to convert the portion of energy into heat, so that the excessive portion of the DC bus voltage may be biased to the resistor and thus various components can be protected.
When more than one braking chopper is involved, a method to control the operations of these braking choppers may be needed. The braking chopper can also be referred to as a chopper unit. One existing method is to control the chopper units individually, which may cause large difference in the chopper unit and resistor load share by small inaccuracies in a measurement circuit having analog components. Another existing method is to set one master chopper unit and additional chopper unit(s), with all of the chopper units being operated simultaneously. By utilizing this method, all of the chopper units may fail to function if the master chopper unit fails. In addition, a high current pulse may occur, which in turn negatively impacts the components connected to the DC link (e.g., in the form of additional thermal stress, distribution to controllers, and the like), because all of the chopper units are switched on and off at the same time. The high current pulses caused by the activations of the chopper units are represented as DC link ripple current, which is the AC component of the current through a capacitor of the DC link (or can be referred to as a DC capacitor).
In view of the foregoing, there is a need in the art for a solution for controlling the chopper units so as to lower the amplitude of the ripple current when the chopper units are activated.